1. Field of the Invention
The present invention relates to a vehicle electro-hydraulic power steering unit that assists steering-wheel operating force of a vehicle to reduce an operating force of the steering wheel.
2. Description of the Related Art
As a conventionally typical power steering unit, a hydraulic type power steering unit and an electric type power steering unit have been known. In most of the hydraulic type power steering units, it is an engine that drives an oil pump. In such a hydraulic type power steering unit, however, the oil pump is driven on a steady basis by the engine even at the time of straight running requiring no assist, so that a problem exists in large loss of engine horsepower. On the other hand, an electric type power steering unit is arranged such that the use of the above-described oil pump can be eliminated, and an electric motor is started only when necessary to be power-assisted, so that an advantage resides in small loss of engine horsepower. Furthermore, as a power steering unit utilizing these oil type and electric type jointly, there is an electro-hydraulic type power steering unit in which a motor is utilized instead of the engine acting as a drive source of generating an oil pressure in the hydraulic type, and power is assisted by drive force of the motor at the time of requiring any assist in steering.
FIG. 24 is a schematic view showing a conventional vehicle electro-hydraulic power steering unit, which is disclosed in, for example, the Japanese Patent Publication (unexamined) No. 270425/1993 (pp 3–4, FIG. 1). In the drawing, reference numeral 1a designates, for example, a rack and pinion type steering gear forming a steering mechanism 1. The steering gear 1a is constructed so as to contain a rack 3 extending in a direction of vehicle width, and a pinion gear 4 that engages with the rack 3 within a casing 2 (it is described in more detail in the Japanese Patent Publication (unexamined) No, 95574/1992 (FIGS. 1–2)). Further, the pinion gear 4 is connected to a rotary shaft 6 of the steering gear la via a torsion bar 5.
One end portion of the rack 3 is connected to a front wheel 10 on the left side via a steering rod 7, a tie rod 8 and a knuckle 9. The other end portion of the rack 3 is connected to a front wheel 10 on the right side via a power cylinder unit 12 (corresponding to a piston mechanism), a steering rod 11, a tie rod 13 and a knuckle 14.
The rotary shaft that protrudes from the top of the casing 2 is connected to a steering wheel 18 via a steering joint 16, and, e.g., two-piece housing type steering shaft 17. In this manner, by the operation (turning) of the steering wheel 18, a displacement is transmitted to the knuckles 9, 14 through the steering shaft 17, the torsion bar 5, the pinion gear 4, the rack 3 and the steering rods 7, 11, and the front wheels 10, 10 on two sides are steered in a direction of being operated. In addition, numeral 17a designates a steering column.
Further, a rotary valve 19 (supply and exhaust valve) is provided between the rotary shaft 6 of the steering gear 1a and the pinion gear 4. In this rotary valve 19, as known (for example, in the Japanese Patent Publication (unexamined) No. 95574/1992 (FIGS. 1–2)), an outer valve 19a having a tubular shape is provided on the pinion gear 4, and an inner valve 19b having a tubular shape that engages with the outer valve 19a is provided on the rotary shaft 6. That is, the rotary valve 19 is arranged such that torsion of the torsion bar 5 causes a relative displacement to occur between the outer valve 19a and the inner valve 19b. 
An inlet port 20 that is formed in the foregoing rotary valve 19 is connected to a discharge part of an electro-hydraulic pump 22 via a passage 21; and an outlet port 23 is connected to an intake part of the electro-hydraulic pump 22 via a passage 24. That is, the electro-hydraulic pump 22 is constructed such that a driving motor 22b is directly connected to a pump section 22a, and a reservoir 22c is connected to the intake part of the pump section 22a; and that the passage 21 is connected to the discharge port of the pump section 22a, and the passage 24 is connected to the intake port of the reservoir 22c. 
The motor 22b of the electro-hydraulic pump 22b is connected to a power supply circuit, which is constituted of a battery and alternator mounted on the vehicle being connected in parallel (any one of them is not shown). The electro-hydraulic pump 22 is brought in operation with an electric power from the foregoing power supply circuit being the source of power. A pair of output port parts 26a, 26b that are formed at the rotary valve 19 are connected to the power cylinder unit 12 via passages 27a, 27b. 
That is, the power cylinder unit 12 is constructed such that a piston rod 28, which is connected to the steering rod 11, runs through a cylinder 29 that is fixed to, e.g., frame of an automobile, and that a piston 30 slidable, partitioning an internal part of the cylinder 29 into right and left (in a direction of vehicle width), is provided on a part of this piston rod 28. Chambers 31a, 31b (pressure-receiving chambers) on both (right and left) sides of the piston that are formed by partitioning with this piston 30 are connected to the mentioned output port parts 26a, 26b via a pair of input port parts 32a, 32b and passages 27a, 27b. 
In the hydraulic circuit 34 of such a construction, when the steering wheel 18 is in a neutral state, an oil pressure that is generated in the electro-hydraulic pump 22 is supplied to both chambers 31a, 31b as a neutral pressure through the rotary valve 19. Then, at the time of the steering wheel 18 being steered, an oil pressure responsive to a supply and exhaust operation of a relative displacement between the outer valve 19a and the inner valve 19b of the rotary valve 19 in accordance with the steering of the steering wheel 18, that is, an oil pressure in accordance with steering (steering wheel force and steering direction) is supplied to the chambers 31a or 31b. The front wheels 10, 10 can be operated while an oil pressure that are generated in the chambers 31a, 31b rendering an assistance to the steering wheel force.
Further, a control unit 37 that is constituted of a microcomputer and peripheral circuits thereof is connected to the motor 22b of the electro-hydraulic pump 22 via the drive circuit 36. Connected to this control unit 37 are a vehicle speed sensor 38 that detects a speed of vehicle, and a steering torque sensor 39 that detects a steering torque of the steering mechanism 1, for example, a steering torque generated at the steering wheel 18. Further, by means of a circuit that is formed by these electronic equipments being connected, a predetermined assist power is generated in the power cylinder unit 12 at the time of steering depending on a vehicle speed and a steer state; and the operation of the electro-hydraulic pump 22 is stopped over all range of vehicle speed at the time of no steering.
Furthermore, as a function of determining whether or not the steering wheel 18 is operated, the control unit 37 performs a function of comparing an output signal (steering torque) Tss from the steering torque sensor 39 and a set threshold (torque threshold) T10 to determine a magnitude therebetween. That is, “steering” is determined when Tss>T10, and “no steering” is determined when Tss<T10. When the steering is determined, the hydraulic driving motor 22b is driven, necessary oil pressure is generated, and an assist is given to steering. Whereas, when non-steering is determined, the operation of the hydraulic driving motor 22b is stopped over all range of vehicle speed. Consequently, power saving of the electro-hydraulic pump is achieved.
As described above, in the conventional vehicle electro-hydraulic power steering unit, the operation of the hydraulic driving motor is stopped at the time of non-steering, thereby achieving power saving of the electro-hydraulic pump. However, it is a recent trend that a further power saving (improvement in fuel consumption) is required.